There is much demand for filters capable of being used in subterranean wells in order to remove particulates from liquids or gases produced by the wells. Typical particulates which need to be filtered out are sand and clay, and for this reason, such filters are often referred to as sand screens. Unconsolidated particulate matter, hereinafter referred to as "formation sand," is often associated with subterranean hydrocarbon formations. A major problem in producing hydrocarbon fluids from unconsolidated formations is the intrusion of formation sand, which is typically very fine, into the production fluid and equipment. The presence of sand in the production fluid often leads to the rapid erosion of expensive well machinery and hardware. Furthermore, the shifting of sand in an unconsolidated formation may result in the collapse of perforations purposefully blasted in the formation, thereby reducing or even halting production. In order to prevent the shifting of formation sand, such formations are typically "gravel packed." The gravel packing also acts as a filter, preventing the fine sand from entering the production fluid. Gravel packing involves the introduction of a particular grade of sand into the well bore, often by pumping it down as a dense slurry, into the annulus defined by the inner circumference of the casing wall and the outer circumference of the work string. The gravel pack is often forced through the perforations in the casing wall and incorporated into the formation perforations, thereby stabilizing the formation.
Since the gravel pack itself comprises sand, sand screen assemblies are utilized to prevent the intrusion of gravel pack sand into the well production fluid. If too much formation sand passes through the gravel pack a collapse of the formation can occur. In such instances the well may need to be re-perforated and repacked, usually at substantial expense.
The production of hydrocarbon fluids from horizontal wells is highly desirable in that the producing zone may be much longer than for a vertical well, thereby increasing the production efficiency from a particular formation. However, horizontal well completion involves several technical impediments, and as a result, has, heretofore, not been extensively practiced. Placing a production pipe deep into the earth and then attempting to snake the pipe at an angle approaching ninety degrees along a hole with a radius of curvature as tight as thirty feet requires pipe made from a material which combines mechanical strength with flexibility and ductility. The problem of placing production pipe in the well is often exacerbated by the fact that many horizontal wells are of an unconsolidated nature. Gravel packing and sand screen use are very difficult under such circumstances.
Various types of sand screens have been designed for preventing sand intrusion from unconsolidated formations. Examples include a wire-wrapped screen assembly (see, for example, U.S. Pat. No. 3,958,634), a wire-wrapped screen and prepacked gravel assembly (see, for example, U.S. Pat. No. 5,050,678) and a sintered metal unitary body assembly (see, for example, U.S. Pat. No. 5,088,554). Wire-wrapped screen assemblies exhibit several undesirable tendencies: erosion induced by fine sand that initially flows past the wire/gravel pack interface; plugging with carbonaceous, siliceous or organic solids; and collapse or gaping of the wire screen due to the effects of formation and geo-pressure.
Prepacked wire-wrapped screen assemblies also suffer, to varying degrees, from plugging and the effects of well bore stresses. Furthermore, many prepacked screens have a substantially larger outer diameter than the production pipes around which they are disposed, making initial placement and retrieval difficult.
Prepacked, and to some degree wire-wrapped, sand screens are not particularly damage resistant; they require very careful handling on the drill rig floor and during placement in the well bore. Even a slight bump from the casing wall may create a gap in the wire spacing which could lead to erosion and failure of the screen. Furthermore, conventional wire-wrapped screens and prepacked screens can develop gaps in the wire spacings during placement in a horizontal well which can lead to a failure in the screen.
Sintered metal unitary sand screens are cost prohibitive for use in all but the most critical situations. Further, the filtration medium of such a sand screen lacks an integral support and is not damage resistant. Homogeneous, or monolithic constructions allow a crack to propagate, via stress concentration effects and low ductility, through the entire unitary body thickness. Sintered metal unitary assemblies also have a tendency to plug from fines entrained in the formation fluid. To enhance their performance, sintered metal unitary sand screens may need to be electropolished which adds to their cost.
While sintered metal unitary sand screen assemblies exhibit an effective open area (voids volume), they lack a mechanism to facilitate uniform flow distribution between the unitary body and the perforated pipe. Poor downstream flow distribution (drainage capability) will, in effect, create flow channels, resulting in higher flow velocity areas, higher pressure drops, and early plugging. Some wire wrapped and prepacked wire screens have an efficient use of flow area and flow distribution, but exhibit very low effective open area (voids volume), which may cause a reduced production rate. Furthermore, so-called "armored" screen prepacks have extremely poor flow distribution.
Damage resistance is an important attribute of a filter for use in wells. Even if a filter can be prevented from damage during installation in a well, due to the enormous pressures to which the filter may be subjected during use subsequent to installation, it is frequently impossible to completely prevent damage to the filter. In fact, when an underground formation collapses or shifts, it is not uncommon for a filter surrounded by the formation to undergo substantial deformation, such as elongation or crushing. Ideally, a well filter should be able to experience large deformations without losing its ability to prevent the passage of particulate matter, but as described above, conventional well filters typically suffer a severe drop in filtering ability even when subjected to even modest deformation.
The problem of reliably removing sand and other particulates from a production fluid is experienced in many types of wells other than oil and gas wells, such as water wells, geothermal wells, and wells for ground remediation.